Dependence of the endurance limit on the character of residual stresses distribution in the surface layer of hardened parts

The influence of the character of compressive residual stress distribution on the endurance limit of surfacehardened specimens made of various steels and alloys under bending has been examined. Specimens made of ВНС40 and 38Х2МЮА steels with V-shaped notches were hardened by nitriding, foundation specimens made of ВКС-5 alloy – by cementation and ionic cementation, flat specimens made of ЭИ698ВД alloy – by airshot-blasting. The influence of surface hardening on the endurance limit was assessed using two criterions, namely: residual stresses on the dangerous section surface and the average integral residual stresses. It was established that the endurance limit of surface-hardened specimens (parts) is determined not by the value of compressive residual stresses on the surface but by the completeness of the diagram of the residual stress over the part surface layer thickness on condition that the surface layer thickness is equal to the critical depth of the nonpropagating fatigue crack. The average integral residual stress criterion calculated over the thickness of a surface layer of the dangerous section equal to the critical depth of the non-propagating fatigue crack gives a rather accurate estimation of the influence the character of compressive residual stress distribution has on the endurance limit of hardened specimens (parts). In practice the estimation of the increment of surface- hardened specimens (parts) endurance limit by the criterion of residual stresses on the dangerous section surface may lead to a significant error.